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Author: Subject: Dean Stark without Dean Stark.... almost quantitative
Tsjerk
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[*] posted on 22-2-2020 at 04:22
Dean Stark without Dean Stark.... almost quantitative


I tried to use a Dean Stark preparation to prepare the diketal of cyclopentanone with ethylene glycol. The first time I did this was with toluene. This worked fine and I got a fine yield of 69%.

The second time I did it in xylene, this time I didn't get any water in the trap but I did get a very obvious water layer at the bottom of the flask. Apparently water is so insoluble in xylene there is no need for a Dean Stark trap.

What I did was refluxing xylene with a bit of sulfuric acid (1 or 2 ml in 200 ml), which turned brown, and separate the acid from the xylene. This xylene was used to prepare cyclopentanone ethylene diketal. A clear water layer formed after less than half an hour (about the amount calculated, 6 ml). The water layer was separated and after washing with brine and drying with magnesium sulfate the xylene was evaporated. A liquid with a boiling temperature well above 150 degrees was isolated.

My best guess is that the liquid is the ketal of cyclopentanone. It is immiscible with water, so I think it can't be ethylene glycol. Neither can it be cyclopentanone nor xylene as the boiling temperatures of those are lower. It weighs 50 grams, which would be a quantitative yield. I will do a vacuum distillation on it after which I will determine the final yield.

Conclusion: there is no need for a Dean Stark trap; refluxing and separation of the water layer is sufficient when using xylene.

The xylene was obtained from thinner, simple distillation was preformed. The label says it contains xylene, acetone, isopropanol and isobutanol. Everything boiling below 135 was disregarded. The boiling flask was clean after distilling; no residue.

[Edited on 22-2-2020 by Tsjerk]
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Metacelsus
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[*] posted on 22-2-2020 at 04:37


Hmm, interesting. The benefit of a Dean-Stark trap is being able to continuously remove the water as the reaction proceeds, but I guess in this case a simple phase separation performs the same role. I wonder if this can be applied to other reactions — if so, it would be useful to those of us who don't have Dean-Stark traps.



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Tsjerk
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[*] posted on 22-2-2020 at 05:07


I'm pretty sure this can be applied to other reactions which ask for a Dean Stark. It should at least work for ketalizations with ethylene glycol of ketones with a boiling point of at least 130 degrees. It should also work for ketones with a boiling point above 92 degrees, as the azeotrope of xylene and water boils at that temperature.

Nowadays xylene is more OTC than toluene is.

Edit: the ketal should boil above the boiling temperature of xylene, of course... not the ketone. The reaction could be run at room temperature if necessary. And even; the ketal could be distilled if boiling below xylene.

[Edited on 22-2-2020 by Tsjerk]
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chemistry007
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[*] posted on 3-3-2020 at 00:32


About dean start setup, one thing that i think can be interessting to try is add 3A molecular sieve.
In that way no need dean stark and all water is sucked by the sieve.
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Tsjerk
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[*] posted on 3-3-2020 at 01:38


A problem with molecular sieves is that they don't handle acids well. Acids will dissolve they sieves. What you propose might work without an acid, but it would take a decade.

In the case of ethylene there is also the problem of dehydration forming polyethylene glycol and I would also not trust sieves around any ketone for that same reason.

[Edited on 3-3-2020 by Tsjerk]
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chemistry007
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[*] posted on 3-3-2020 at 01:54


SWIM using it with ketone...specially with methylamine.
But you are probably right, in acid condition it's not that usefull...but why you think it can take decade? As long as water is formed, it's sucked by the seave...just have to have good stirring.
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[*] posted on 3-3-2020 at 05:01


You don't have to "swim" here, better don't.

The reason you can use it for your condensation is because there is methylamine around, but one of your side products is a condensation product of the ketone with itself, that is why you shouldn't dry for example acetone with sieves.

The reason why I think it would take a decade is because the reaction needs an acidic catalyst, without it either doesn't run or it takes ages.
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[*] posted on 3-4-2020 at 11:56


I tried anilation of a piperidone in xylene and the yields were less than disappointing ;^(
A well calibrated Dean Stark is a good way to monitor reax progress too. The apparatus is not expensive. I like to use it in conjunction with a constant temperature control. I suppose sieves would work but there would be no way to observe the evolved water if the water is a measurement parameter. Also sieves complicate the workup and add to the expense.

[Edited on 3-4-2020 by chemrox]




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